The ever-decreasing feature size of new semiconductor devices has resulted in demand for increasingly high precision projection lenses for microlithography. To produce higher precision projection lenses, it is necessary to characterize performance, i.e., aberrations, of the lenses with higher accuracy.
Conventional performance evaluation of an i-line projection lens (a projection lens used in a microlithography device employing a mercury-vapor i-line light source) or an excimer projection lens (a projection lens used in a microlithography device employing an excimer laser light source) is conducted, for example, by a method in which a certain spatial test pattern is imaged through the lens to be tested. The performance of the test lens is evaluated based on the image of the test pattern produced by the lens compared to a standard.
This conventional method for evaluating i-line and excimer projection lenses poses certain problems. Because the test lens performance is evaluated based on imaging performance, i.e., by a spatial image measurement, aberrations of the test lens are not directly measured. Therefore, the performance or aberrations of the test lens cannot be determined with high accuracy. Without an accurate determination of the performance or aberrations of the test lens, it is impossible to obtain a computer simulation, based on the performance of the test lens as tested, with parameters such as illumination, spatial image characteristics, and image height being freely varied. Since the performance of the test lens cannot be measured with high accuracy, an indicated value for gap adjustment cannot be calculated. In other words, information required to calculate an adjustment for lens correction cannot be obtained. Thus a significant amount of time is required (for trial-and-error adjustments) to produce a lens having a desired performance.
A representative conventional apparatus for evaluating excimer projection lenses is disclosed in Japanese Laid-Open Patent Publication No. 1-134224. The disclosed apparatus includes: a narrow-band excimer laser; an interference optical system including means for splitting light from the excimer laser into two beams that traverse different optical paths having an adjustable path difference, and an image-pickup means for receiving an image of an interference pattern formed by recombining the beams; and a processing device for receiving information from the image-pickup means and calculating wavefront aberrations of a test lens placed in one of the split optical paths in the interference optical system.
Measuring wavefront aberrations of a test projection lens to a high degree of accuracy is difficult with the above-described proposed apparatus.